In the old times, the whole idle task was consideredas an RCU quiescent state. But as RCU became more andmore successful overtime, some RCU read side criticalsection have been added even in the code of somearchitectures idle tasks, for tracing for example.

So nowadays, rcu_idle_enter() and rcu_idle_exit() mustbe called by the architecture to tell RCU about the partin the idle loop that doesn't make use of rcu read sidecritical sections, typically the part that puts the CPUin low power mode.

This is necessary for RCU to find the quiescent states inidle in order to complete grace periods.